A mobile station network is a telephone network that is based on radio-techniques, that consists of radio-base stations and radio and wire connections between the base stations, and terminals and mobile stations centrals and services contained in the centrals. A base station is in such a mobile phone network a component to which, the mobile connects when in use. These base stations are connected to each other either by radio link or by a wired network. The terminal device is for example a mobile phone, which is a portable phone terminal using radio communication. Technically the mobile phone may be a phone using e.g. GSM or GPRS-techniques. The mobile station central on the other hand is a phone switch, that directs the phone terminal device to the right channel for receiving a call. Mobile communications is then wireless communication between the telecommunications network (base stations) and the terminal devices (mobile phones) by using terminal devises utilizing radio communications.
When the user makes a call or sends a message with his mobile phone, the message is first transformed to digital form and is then transferred wirelessly over radio frequencies, usually to the nearest base station. The base station contains cells that form the coverage area of the base station. The form of the cell is determined by in which direction and at which height the antenna of the transceiver unit on the base station is installed and in what kind of terrain the base station is situated. Typically the antennas of the base stations tend to be placed as high as possible in the countryside and at the roofs and walls of buildings in cities. This way geographical obstructions hinder the course of the radio signal, i.e. the wireless transmission of the call or the message, as little as possible.
Today mobile phone networks are based on cell systems, where transmission power is not increased in order to expand the coverage area, but the operation is based on reuse of the frequencies. The same frequency is used in many places, which are though at sufficient distances from each other, not to interfere with one another's transmissions. There is at least one transceiver a cell of a base station. The mobile phone network also comprises a base station controller, which controls the traffic of the base stations, i.e. forming of, or ending calls or data connections, signaling and the use of radio frequencies. There are several base stations in the range of one controller. From the base station the call or message goes to the mobile station central, which relays the call or message from the mobile station network to another or to other telecommunications networks. If the call ends up at another mobile station, the message travels the last part wirelessly via radio frequencies. Such a telecommunications distance for a call from a phone to another is thus very long and complex, even though the call makes the distance in a fraction of a second.
The GSM-network is for example not closed, but it has connections also to the world outside the network. The GSM-network can though, be divided into parts according to how it is connected to this external world. This kind of division is takes place by means of the inner structure of the GSM-network and its connections to the outer world. Connection of the users' mobile phones to the GSM-network takes place through the base station subsystem (BSS). The operator monitors and controls the network with the administration subsystem (OSS) and the connection of the GSM-network to other networks and the connection of the network is a task of the network and connection subsystem (NSS).
The BSS is responsible for the radio path control and its task is to connect the mobile phones to the mobile phone central. The BSS is further divided into two parts, i.e. to base stations (3TS) and base station controllers (3SC). The base stations comprise needed transmission and receiving devices, to enable a signal to travel from the mobile phone to the GSM-network over radio paths. The base stations are the core of the cell network, since every base station form a cell of its own. The control of the radio paths is centralized in the base station controllers, which can control several base stations. Main tasks of the base station controllers are among others reserving and freeing radio channels and controlling the handover of the base stations.
The most essential component of the NSS is the mobile station central (MSC), which is responsible for connection of the calls inside the GSM-network and between the GSM-network and external networks. In principle, the functionality of the MSC largely corresponds to the functionality of the central in the fixed network, since in the same way it connects the time slots of the incoming PCM (pulse code modulation)-connections to the time slots of the outgoing PCM-connections as the central in the fixed network. In order to be able to connect calls to mobile stations, the central needs to know where the mobile station in question is situated at that time. This information is saved in the common home location register (HLR) for all the centrals. HLR is a database where, additionally to common subscriber information also information about the approximate position of a single mobile station is maintained. More exact information on the position is saved in the MSC-specific visitor location register (VRL). The VLR is responsible for temporary saving of the position information for the subscribers on the service area of each of the MSC's. There can be several mobile station centrals in a mobile station network. Since an outside caller does not have information on in the area of which central the mobile station is at a given moment, the call has to be routed into the mobile station network without any kind of position information. This task, of receiving calls coming from other networks, is performed by the gateway mobile services switching centre (GMSC). The GMSC thereafter attends to that the call is directed to the right mobile station.
OSS takes care of that the mobile station network operator is capable of controlling its own network. The OSS assembles the different parts of the mobile station network to one controllable unit. The control procedures can be divided into bug control, configuration control, billing control, performance control and security control. The OSS offers different possibilities for management of all these sectors.
GSM then allows connecting of different voice and data services and operations between networks. GSM has defined three different services: network, tele- and support services. The mobile station is connected to the GSM PLMN (infrastructure needed by the GSM) network through an interface. The network is connected to transfer networks, for example to an ISDN or a traditional PSTN. There can further be some additional network in between (source or target network) before a station forms a connection. The network service gathers all services, which enable transparent data transfer via the interfaces to the network. If interfaces have not been determined for all networks, the network determines which interfaces can be used in transparent data transfer. In the traditional GSM-model network services are connectable and circuit and packet switched. These services require only the three is lowest levels of the ISO/OSI-model. In the mobile station, the mobile terminal performs all the network specific tasks (e.g. TDMA, FDMA and coding) and provides an interface for data transfer to another terminal, which can be independent of the network. Depending of the capabilities of the other terminal, other interfaces may be needed. Tele services are application specific and may thus need all of the layers in the ISO/OSI-model. Those services are defined from point to point i.e. from one terminal to another.
GSM defines many different mechanisms for data transfer. Network services allow transparent and non-transparent and synchronized and non-synchronized data transfer. The transparent network services use only the functions of the physical layer for sending data. The data transfer has, in other words a constant delay and transit, if no transfer errors occur. The only way to improve the quality of the transmission is to use FEC-correction, which codes a repetition into the data flow and helps reforming of the original data in case of a transfer error. Transparent network services do not try to restore lost data, in the case of for example shading or an interruption occurring while change of base station. Non-transparent network services use the protocols of layers 2 and 3 in order to be capable of carrying out corrections and flow control. These services use transparent network services, to which they add the radio link protocol (RLP). There are many network services defined by transparent and non-transparent services in GSM, which are used with the PSTN, ISDN and packet switched public networks, such as the X.25 that is used all over the world. The data transfer can be full duplex and synchronous on data transfer rates of 1.2, 2.4, 4.8, 9.6 kb/s or fill duplex and asynchronous on rates of 300-9600 b/s.
GSM focuses mainly on voice centered telecommunication services. They are transmission of encrypted sounds, message services and basic data transmission, which is known already from PSTN and ISDN. Since the main service is still the phone, the main object was to produce high quality digital voice transmission and to offer at least the same bandwidth of 3.1 kHz as the normal analogue phone systems. For this task, specially designed coders and decoders are used for the voice transmission, while other coders are used for analogue transmission for traditional computers.
Additionally to network services the GSM service providers may offer other support services. According to ISDN networks these services in many ways improve common phone services. The offered services may vary from one service provider to another. Typical services are user identification and directing of calls i.e. rerouting of incoming calls. Further ISDN-features are available, such as closed user groups and conferences with several attendants.
GSM is a hierarchical and complex system architecture with many units and interfaces. The GSM-system is thus formed by three subsystems: the RSS, Radio SubSystem, the NSS, Network and Switching SubSystem, and the OSS, Operation SubSystem. Usually the user notices only a small part of the whole network, i.e. the mobile stations and some of the base station transmitter antenna masts.
RSS or the Radio SubSystem is formed by the units linked to radio, i.e. the mobile stations and base stations. The GSM-network is formed by many subsystems of base stations, each controlled by a base station controller. The base station carries out all maintenance functions of the radio connections of the base stations, codes and decodes sounds and adapts the speed to the wireless part of the network and vice versa. Additionally to the base station controller the base station comprises several transmission/receiver units of the base station. The transmission/receiver unit of the base station is formed by all the devices needed for radio transmission: antennas, signal processors and amplifiers. The unit can form a radio cell, or using directional antennas, several radio cells. It is connected to the mobile station via a first interface and to the base station controller via a second interface. The first interface comprises all the mechanisms needed in wireless transmissions (TDMA, FDMA etc). The second interface is formed by 16 or 64 kb/s connections. A GSM cell can be in size range 100 m-35 km depending of the surroundings and the expected traffic. The base station controller in fact controls the trans mission/receiver units of the base station. It reserves radio frequencies, handles the exchange of the transmission/receiver units in the base station and implements the searches made by the mobile stations. The base station controller also channels the radio channels to fixed network connections at the interface. A mobile station is formed by the user's all devices and programs needed for communication in a GSM-network. A mobile station is formed by all the user's devices and programs and the subscriber's identification module, i.e. the SIM-card, onto which all the user specific data is saved. User specific mechanisms, like charge and verification, are based on the SIM-card and not on the actual device. Device specific mechanisms like theft inhibition, use device specific identifiers.
NSS i.e. the network and the switching central for calls is the core of the GSM-system. NSS connects the wireless network to standardized public networks, switches base stations, comprises functions for globally locating users and supports charging of the users, collecting of chargeable events and roaming of operators all over the world. The NSS is formed by switching centrals and databases. The switching centrals for mobile services are very efficient ISDN-switching centrals, which form connections to other mobile phone centrals and base station control centrals through an interface. These centrals form the fixed main trunk of the GSM-system. Usually, the mobile phone central administers several of the base station control centrals in the same area. The Gate Way mobile phone central has additional connections to other fixed networks, like the PSTN and ISDN-networks. Using conciliation measures the mobile phone central can also connect to public telecommunications networks, such as the X.25-network. The mobile phone central provides for all the needed signaling, disconnecting of the connection and assignment to other mobile phone centrals. The mobile phone central also performs all the functions of additional services, such as forward direction of calls, phone conferences with several attendants and collect calls. The home register is the most important database in the GSM-system, because all the essential information of the users is stored therein. They are static data, such as the ISDN-number of the mobile subscriber, subscribed services and the verification key. Further, dynamic data is needed, for example the current position of the mobile station. Immediately when the mobile station moves from its current position area, the information is saved in the home register. All user specific data occur only once in the individual home register, which also supports charging and gathering of chargeable events. The visitor register of the mobile phone central is a very dynamic database. All needed information on the users of the mobile stations, which are currently on the position area of the mobile phone central, is stored therein. If a new mobile station enters the position area, of which the visitor register is responsible, it copies the needed information on the user from the home register. Due to this visitor and home register hierarchy, it is not necessary to continuously update the home register, and no long distance transmission of user information is needed.
The third part of the GSM-system, i.e. the operation and maintenance central (OSS) comprises all functions relating to the function and maintenance of the network. OSS controls its own sections of the network and handles other sections with SS7-signalling. OSS comprises the administration central, verification central and the device register. The administration central observes and controls all other parts of the network through an interface. Common service functions are tracing of traffic, status reports of the network parts, control of the subscribers and safety and gathering of chargeable events and charging. The administrative centrals use a standardized telecommunications administration network. The verification central is determined since the radio interface and the mobile stations are very vulnerable, and for user identifying and for protection of data transfer. The verification central has algorithms for verification and keys for encryption. The verification central creates the values that need verification from the user in the home register. The verification central can in principle be situated in an especially protected part of the home register. The device register is a database for all the IMEI-identifiers. It contains identifiers for devices registered in the network.
TETRA is a GSM-like digital network closed from the general public. The up-to-date features of the system ensure, in addition to a high data security high quality speech and data characteristics. TETRA functions according to the principle known from trunking networks. Thus the network's resources are available to all groups connected to the network. Additionally the TETRA-terminals support direct mode (DM), so no network is needed on this connection interval. The latter practice reminds of a type of LA-type call, even though TETRA enables for example very good protection against outside listeners.
TETRA is like GSM based on TDMA-techniques (Time Division Multiple Access). The TETRA-frequencies are divided into frames formed by four intervals, which are repeated according to the same principle as in GSM. Using TETRA the sending party has to push a button in the device to be able to transmit in half-duplex.
The frequency period is 25 kilohertz in TETRA, while it is 200 kilohertz in GSM. As modulation technique TETRA uses DQPSK (Differential Quadrature Phase Shift Keying). The rate of speech coding is 7.2 kilo bits per second including error correction. The speech service can be used as half duplex or full duplex, depending on the situation.
The interface of TETRA is named AI (Air Interface). Radio interface may be used either between the mobile station and the network or in direct connections between mobile stations. The interface between the TETRA-base stations and centrals can be implemented with a 64 kilobit per second PCM transmission that is known from GSM, thus so that single channels of the radio interface are under multiplexed into 8 kilobit blocks per second. The capacity between the base station and the central can thus in principle be used two times more efficiently compared to the 16 kilobits under multiplexed blocks of the Abis interface in GSM.
When a TETRA-connection is formed between the user and the network, a user identification and encryption if the interface is performed according to very similar principles as in GSM. The difference from GSM is additionally to the difference in identification and encryption algorithms are that the TETRA network offers the possibility of mutual authentication. Thus the user of the terminal device can ensure that he uses an authentic TETRA network. When two TETRA networks are connected to each other, the Inter-System Interface (ISI) is used. Thus TETRA offers in principle a possibility also a type of network roaming, just like in public GSM-networks. It is possible to connect external devices to the TETRA-stations, for example computers and terminals through the Peripheral Equipment Interface (PEI). The terminals enable among other things data transmission between the devices and the network.
An important element of the TETRA-network is a so-called service central. This means a place, where the messages are physically received. The service central is connected to the central system either directly or through a separate transit network (PSTN, ISDN, PDN, or the like). The interface of the service central is named LSI (Line station Interface).
Also connections to outside networks are defined in the TETRA-system, such as the public switched telephone network (PSTN), the ISDN-network or a packet switched network (PDN, Packet Data Network). As in GSM, a separate network control element is needed in TETRA for maintenance and control that is connected through the NMI-interface (Network Management Interface) to the central system. This is the only optional interface in TETRA. It is possible to connect to the network management for example via a separate terminal.
A TETRA call can be formed either through the network or directly between the users (Direct Mode). The latter means in practice that at least two users can form a connection directly between their terminal devices thus that the network is not in any kind of connection to the users. According to the Direct Mode, the sender and receiver have in communication one common frequency in use, on the time slots of which the direction of sending and receiving is alternated. In a single time slot of TETRA the sending party is a so-called master and the receiving party is a slave. In a normal TETRA emission two pieces of time slots of a singe frequency for each transfer direction can be used. A special frequency efficient mode is also defined in TETRA, where for each single frequency two duplex calls can be transmitted.
Direct mode is usefull especially when the coverage area of the normal trunking-network is missing or the signal strength of the network is weak. On the other hand direct mode frees capacity to the network, so the function can be used also in a kind of load distribution. All in all direct mode is such that the connections are formed at a single frequency and locally, unlike when communication occurs through the network.
In data transmission exactly the same principles are used in TETRA as in the HSCSD (High Speed Circuit Switched Data) or GPRS (General Packet Radio Service) in GSM, i.e. there is a data transmission of variable rate according to the used channel coding and the number of time slots. For parallel time slots the term multi slot can also be used in TETRA.
Data transmission in TETRA can be implemented with so called Bandwidth on Demand. When needed more capacity than normally can be given to a single terminal for example for the time of the transmission and receiving of information including pictures. Additionally to circuit switched data transmission packet switched connections (connectionless and connected X.25-communication) is defined in TETRA. TETRA also supports protocols IPv4 and IPv6. In packet switched mode outside packet switched networks see TETRA in the same way as any other IP-sub network or X.25-node.
Automatic handover is defined in TETRA according to the same principles as in other modern cell networks. Because of this trunking-connections can continue even though that the signal strength of original node used in the beginning of the connection turns insufficient.
Like GSM TETRA has channels and separate control channels (for confirmation of access, allocation of traffic channels and control of traffic channels) reserved for the data transfer of the radio interface. The traffic channels can be used so that the users of priority can access the system before the users of lower priority. The priority function can be used also so that already existing calls may be interrupted in order to allow a call of higher priority to enter the system. In the priority function the specifications are though left to be implemented to the device manufacturers for example when it comes to the priority between data and speech traffic.